(1) Field of the Invention
The present invention relates to an electroless plating apparatus, an electroless plating method, and a manufacturing method of a printed circuit board.
(2) Description of Related Art
In general, electroless plating is the process of depositing metals on the surface of an object to be plated by a reduction reaction without any electric current applied, in which a catalyst is attached on the surface of the object which is then immersed in an electroless plating solution. The electroless plating also allows plating of the surface of an insulting member with a metal film. Thus, the electroless plating has been widely used in the industry.
In recent years, various types of electronic equipment employ high-density and high-fineness printed circuit boards. In manufacturing such printed circuit boards, a metal thin film of nickel, chromium, or the like is formed on the surface of a wiring trace of copper by the elecctroless plating. In this case, the metal thin film can also be formed on very small conductive portions and insulator portions where establishing conduction is difficult.
In contrast to electroplating, the growth rate of the metal thin film is slow in the elecctroless plating, but as thickness variations within the surface is small. Therefore, the elecctroless plating is useful for providing a uniform metal thin film that does not require a large thickness.
JP 4-152261 A describes an electroless plating deposition rate measuring apparatus that measures a deposition rate of an electroless plating solution for the optimization of the thickness of the metal thin film formed by the electroless plating. The electroless plating deposition rate measuring apparatus measures a polarization resistance by periodic application of a voltage between the electrode pair in the electroless plating solution, and calculates the deposition rate of the electroless plating solution based on the measured polarization resistance. JP 4-152261 A describes that, by the use of the calculated deposition rate, the thickness of the metal thin film formed by the electroless plating is controlled to be an optimized value.
When the object is immersed in the electroless plating solution in the presence of a reference electrode in the electroless plating solution, a potential difference of about −450V occurs between the object and the reference electrode. This potential difference comes to a steady state at about −950V after a transient time of about several tens of seconds has passed. In this state, a chemical reaction of the plating process is started.
However, the transient time is affected by several factors including components, temperature, and an index of hydrogen ions of the electroless plating solution. In this context, an electroless plating apparatus described in JP 1-275771 A includes a first electrode which is in contact with the electroless plating solution, and a second electrode which is in contact with the object. A voltage of −950V is applied to the second electrode for two seconds from the stable power supply. A chemical reaction of electroless plating is thus forced to begin. As such, plating time is controlled.